This invention relates to rigid polyisocyanurate cellular compositions and to processes for preparing them using a combination of catalysts.
As is well-known, the polyurethane foams, particularly the rigid polyisocyanurate foams, are excellent thermal insulators in a wide variety of applications, especially in the refrigerant and construction applications. In the production of such foams, the cellular polyurethane is made by reacting an isocyanate with a polyol or other active hydrogen compound having a plurality of active hydrogen groups. Generally, such reactions were carried out in a formulation also containing a blowing agent and a surfactant in addition to the aforementioned reactants and a catalyst. By proper choice of isocyanate and polyol, foams can be made with properties ranging from flexible to semi-rigid to rigid.
Unfortunately, the flammability and smoke evolutionary characteristics of polyurethane foams have significantly limited use of such foams in many applications. Conventionally, the tendency of such foams to burn is reduced by the addition of halogenated organic materials or combinations thereof with phosphorus compounds to a polyurethane formulation. More recently, polyurethanes having improved flame and heat resistance (hereinafter called "thermal stability") have been made by introducing more isocyanurate groups into the polyurethane network. In addition to high thermal stability, such isocyanurate foams also exhibit hydrolytic and dimensional stability.
Such foams are normally produced by the employment of so-called trimerization catalysts in combination with conventional urethane catalyst in the urethane formulation. Exemplary trimerization catalysts include amine catalysts such as 2,4,6-(N,N-dimethylaminomethyl)phenol, hexahydrotriazenes, metal alkoxides, metal carboxylates and the like. Unfortunately, relatively large amounts are required to produce the desired trimerization required for the preparation of isocyanurate foams. The presence of such larger amounts of catalysts, particularly the amine or metal catalysts, often cause degradation when the resulting foam is subjected to elevated temperatures. Some catalysts, particularly the alkali metal carboxylates, yield foams which are so friable that they exhibit little, if any, resistance to impact and abrasion. More recently, as taught in U.S. Pat. No. 4,111,914, isocyanurate foams exhibiting reduced friability have been prepared from stable isocyanurate oligomers formed in the presence of sulfonium zwitterions. While such isocyanurate foams exhibit many desirable properties as compared to conventional isocyanurate foams, further improvements in reaction rates, processability, impact and abrasion resistance are desired.
In view of the aforementioned difficulties with the conventional catalysts employed and the reactions of isocyanates, it is highly desirable to provide a process for preparing polyurethane foams containing substantial isocyanurate moieties, wherein such foams exhibit good thermal resistance as well as good control of impact and abrasion resistance. In addition, it is desirable that improvements in reaction rates and processability be achieved by such process.